Hydrosilylation chemistry, involving the reaction between a silyl hydride and an unsaturated organic group, is the basis for synthetic routes to produce commercial silicone products such as silicone surfactants, silicone fluids and silanes as well as many addition cured products including sealants, adhesives, and silicone-based coatings. Heretofore, hydrosilylation reactions have been typically catalyzed by precious metal catalysts, such as platinum or rhodium metal complexes.
Various precious metal complex catalysts are known in the art. For example, U.S. Pat. No. 3,775,452 discloses a platinum complex containing unsaturated siloxanes as ligands. This type of catalyst is known as Karstedt's catalyst. Other exemplary platinum-based hydrosilylation catalysts that have been described in the literature include Ashby's catalyst as disclosed in U.S. Pat. No. 3,159,601, Lamoreaux's catalyst as disclosed in U.S. Pat. No. 3,220,972, and Speier's catalyst as disclosed in Speier, J. L, Webster J. A. and Barnes G. H., J. Am. Chem. Soc. 79, 974 (1957).
Although these precious metal complexes are widely accepted as catalysts for hydrosilylation reactions, they have several disadvantages. One disadvantage is that the precious metal complex catalysts are inefficient in catalyzing certain reactions. For example, in the case of hydrosilylation of allyl polyethers with silicone hydrides using precious metal complex catalysts, use of an excess amount of allyl polyether, relative to the amount of silicone hydride, is needed to compensate for the less-than-ideal selectivity of the catalyst in order to ensure complete conversion of the silicone hydride to a useful product. The excess polyether which is mostly isomerized, is also most often impractical to remove and remains in the product, and may result in undesirable color and odor in end applications.
While the above-mentioned precious metal catalysts, particularly those based on platinum, are widely used in the industry for their high activity and ease of preparation and handling, these catalysts also suffer significantly from poor selectivity towards the desired adducts, especially for olefins containing hetero-atoms such as oxygen and nitrogen close to the C═C unsaturation as well as certain multiply unsaturated substrates. Examples of the latter are terminally unsaturated dienes such as butadiene, isoprene and others, where the desirable product is a 1,2-adduct that leaves a second terminal unsaturation for convenient further derivatization and application chemistries. Historically, catalysts for hydrosilylation of 1,3-dienes produce essentially the 1,4-adducts or a mixture of 1,4- and 1,2-adducts at best where the internally unsaturated 1,4-adduct predominates or the overall reaction is inefficient. Ritter has reported Fe catalysts that selectively hydrosilylate conjugated dienes such as myrcene to form the 1,4-addition product (Ritter, T. et al J. Am. Chem. Soc. 2010, 132, 13214). Palladium and Rh catalysts (Wilkinson's catalyst) are known to hydrosilylate isoprene and myrcene to give the 1,4-addition product (I. Ojima and M. Kumagai J. Organomet. Chem. 1978, 157, 359-372).
Rhodium(III) acetylacetonate is known to hydrosilylate isoprene to give the 1,4-addition product (M. Lappert et al, J. Organomet. Chem. 1979, 172, 153-163). Cp2TiF2 is known to hydrosilylate isoprene to give the 1,4-addition product (C. Moise et al; Organometallics 2005, 5802-5806). H2PtCl6 is known to hydrosilylate isoprene to give a mixture of products (R. Roche J. Organomet. Chem. 1978, 156, 235-244). While many non-precious metals have also been investigated for 1,3-diene hydrosilylation, essentially all are known to provide either mixtures of the 1,2- and 1,4-adducts or are inefficient in producing the 1,2-adduct (B. Marciniec., Ed. Comprehensive Handbook on Hydrosilylation; Pergamon Press, 1992). Recently, Ritter, et al. reported highly selective 1,2-hydrosilylation of butadiene and other terminally unsaturated 1,3-dienes using a cyclometalated platinum complex containing a phosphine ligand (Ritter, et al. J. Am. Chem. Soc., 2014, 136 (13), pp 4857-4860.)
In addition to the less-than-desirable selectivities or regio-selectivities, due to the high price of precious metals, the precious metal-containing catalysts can constitute a significant proportion of the cost of silicone formulations. Recently, global demand for precious metals, including platinum, has increased, driving prices for platinum to record highs, creating a need for effective, low cost replacement catalysts.
As an alternative to precious metals, recently, certain iron complexes have gained attention for use as hydrosilylation catalysts. Illustratively, technical journal articles have disclosed that that Fe(CO)5 catalyzes hydrosilylation reactions at high temperatures. (Nesmeyanov, A. N. et al., Tetrahedron 1962, 17, 61), (Corey, J. Y et al., J. Chem. Rev. 1999, 99, 175), (C. Randolph, M. S. Wrighton, J. Am. Chem. Soc. 108 (1986) 3366). However, unwanted by-products such as the unsaturated silylolefins, which result from dehydrogenative silylation, were formed as well.
A five-coordinate Fe complex containing a pyridine di-imine (PDI) ligand with isopropyl substitution at the ortho positions of the aniline rings has been used to hydrosilylate an unsaturated hydrocarbon (1-hexene) with primary and secondary silanes such as PhSiH3 or Ph2SiH2 (Bart et al., J. Am. Chem. Soc., 2004, 126, 13794) (Archer, A. M. et al. Organometallics 2006, 25, 4269). However, one of the limitations of these catalysts is that they are only effective with the aforementioned primary and secondary phenyl-substituted silanes, and not with, for example, tertiary or alkyl-substituted silanes such as Et3SiH, or with alkoxy substituted silanes such as (EtO)3SiH.
Other Fe-PDI complexes have also been disclosed. U.S. Pat. No. 5,955,555 discloses the synthesis of certain iron or cobalt PDI dianion complexes. The preferred anions are chloride, bromide and tetrafluoroborate. U.S. Pat. No. 7,442,819 discloses iron and cobalt complexes of certain tricyclic ligands containing a “pyridine” ring substituted with two imino groups. U.S. Pat. Nos. 6,461,994, 6,657,026 and 7,148,304 disclose several catalyst systems containing certain transitional metal-PDI complexes. U.S. Pat. No. 7,053,020 discloses a catalyst system containing, inter alia, one or more bisarylimino pyridine iron or cobalt catalyst. However, the catalysts and catalyst systems disclosed in these references are described for use in the context of olefin polymerizations and/or oligomerisations, not in the context of hydrosilylation reactions. U.S. Pat. No. 8,236,915 discloses manganese, iron, cobalt or nickel complexes containing terdentate pyridine diimine ligands and their use as efficient and selective hydrosilylation catalysts.
There is a continuing need in the hydrosilation industry for highly 1,2-regioselective hydrosilylation of conjugated 1,3-dienes. The present invention provides one answer to that need.